Electron beams (cathode rays) emitted for sterilizing cellular microorganisms collide with metallic components constituting a sterilizer, for example, a stainless holder for holding a container, and thus the electron beams are attenuated and generate braking X-rays or characteristic X-rays. The X-rays are spread widely and then are reflected and diffracted. For example, X-rays having been collided with a shield three to four times can be attenuated to intensity that does not affect a human body, depending on the intensity of X-rays.
It is known that the direction of an electron beam is curved by the effect of a magnetic field. When the outer surface of a container is sterilized by electron beam irradiation, electron beams are widely emitted to the container from an electron beam generator through a wide exit window. Thus, the influence of geomagnetism does not need to be taken into consideration. In a recent technique, electron beams are passed through the wall surface of a container so as to sterilize the inner surface of the container. An electron dose is kept at a predetermined value or less because an excessive electron dose may cause alteration, coloring, or odor on the container. Thus, the recent technique is applied to only a container having an extremely thin wall surface so as to suppress an electron dose. Moreover, Patent Literature 2 discloses a technique for introducing and emitting electron beams from the mouth of a container. In order to prevent irradiation of electron beams around the mouth of the container, an irradiation blocking plate is provided around the mouth of the container.
For example, in a recent technique proposed in Patent Literature 1, an electron beam irradiation nozzle is inserted into a container from the mouth of the container and then electron beams are emitted to the inner surface of the container from an exit window on the distal end of the irradiation nozzle so as to sterilize the inner surface. In the technique for sterilizing the inner surface with the irradiation nozzle, electron beams emitted from the exit window of the irradiation nozzle are spread but a suppressed electron dose limits a sterilization range, requiring the exit window to move close to the bottom of a container. Thus, the length of the irradiation nozzle needs to be nearly equal to the height of the container. A tall container requires an irradiation nozzle having a length of 30 to 40 cm. Moreover, the irradiation nozzle needs to have an outside diameter so as to be loosely fit into the mouth of the container and requires a cooling structure for cooling heat generated by electron beams. This limits the inside diameter of an electron beam passage to about 10 mm in the irradiation nozzle.
The effect of geomagnetism on the above irradiation nozzle may curve electron beams passing through the electron beam passage and the electron beams are emitted from a biased portion of the exit window, or may collide electron beams with the inner surface of the electron beam passage of the irradiation nozzle before reaching the exit window, resulting in abnormal sterilization.
An experiment has proved that the generation of a magnetic field twice to triple ordinary geomagnetism of about 0.3 (Gauss) may collide electron beams with the inner surface of the irradiation nozzle. Moreover, the direction and intensity of geomagnetism drastically vary depending on solar activities including a magnetic storm, a position on the earth's surface, and a time. Furthermore, the direction and intensity of geomagnetism are considerably affected by a magnetic field parallel to the earth's surface from the north pole to the south pole. The influence of geomagnetism on electron beams is sufficiently greater than that of a magnetic field of a motor for transporting a sterilization equipment container. In the case of rotary type sterilization equipment that sterilizes the inner surface of a container during turning along a circular path, the direction and intensity of geomagnetism are found to greatly vary on the circular path. An experiment has proved that the intensity of geomagnetism is changed by about 1 (Gauss) or more, leading to the possibility of unstable sterilization by electron beams emitted from an irradiation nozzle.